The invention relates to a charge-coupled device comprising a semiconductor body having a charge-transport channel, situated at a surface, in the form of a zone of a first conductivity type, which is provided in a well of the opposite, i.e. the second conductivity type, a row of electrodes in the form of conductor tracks being provided above the charge-transport channel, said conductor tracks being separated from the underlying semiconductor body by an intermediate dielectric layer, the row of electrodes comprising at least a first electrode and an adjacent second electrode which, viewed in the direction of the charge transport, is situated behind the first electrode. The invention also relates to a method of manufacturing a charge-coupled device, wherein masked doping is employed to provide a semiconductor body, at a surface, with a zone of a first conductivity type forming a charge-transport channel, and with a well of the opposite, i.e. the second conductivity type, which extends from the surface deeper into the semiconductor body than the zone and surrounds said zone inside the semiconductor body.
Charge-coupled devices belong to a generally known type of integrated circuits and are used on a large scale, for example as two-dimensional image sensors, in cameras to convert an electromagnetic radiation image into a series of electrical signals whose size is a measure of the local light intensity. The image sensor generally comprises a recording matrix having vertical charge-transport channels and a horizontal readout register whose output is connected to an output amplifier. In a known embodiment, the semiconductor body comprises an n-type substrate which is provided at the surface with a p-type well wherein the n-type charge-transport channels are formed.
Charge-coupled devices cannot only be used as image sensors but also to process signals. Consequently, although the invention is important to image sensors in particular, it is not limited thereto.
For a satisfactory operation of charge-coupled devices, it is important that electric charge can be rapidly transported, and substantially without leaving residual charge, from one charge-storage location to a next charge-storage location. This can be achieved by designing the device in such a manner that charge transport takes place under the influence of an electrical drift field. To this end, the length of the electrodes (i.e. the dimension of the electrodes viewed in a direction parallel to the transport direction) is often reduced to a minimum. However, this is not always possible. For example, in the case of an FD output (floating diffusion output), it must be possible to store the total charge below the electrode that was clocked last. This requirement imposes limitations on the minimum value of the effective surface area of the electrode that determines the charge-storage capacity. To increase the width of the electrode (i.e. the dimension transverse to the transport direction) it is necessary to increase the capacity of the floating output zone, which leads to a reduction of the sensitivity of the output. Increasing the length of the electrode has the disadvantage that electrical drift fields below the electrode, which lead to an increase of the charge transport rate, become weaker or disappear altogether, as a result of which a less efficient charge transport is obtained.
Apart from a small electrode length, also other ways, that are known per se, can be used to induce drift fields into the charge-transport channel. For example, in patent document U.S. Pat. No. 5,164,807 a description is given of an image sensor having three readout registers, wherein the transport between the registers is improved by embodying electrodes so as to be more or less conical in shape. However, such a solution is often impossible, for example in the above-described FD output. It has also been proposed to profile the doping concentration in the charge-transport channel in the transport direction. However, this requires additional process steps causing the manufacture to become more complex and hence more expensive.
It is an object of the invention to provide, inter alia, a method of forming drift fields in the charge-transport channel without additional process steps and/or without substantially changing the shape of the electrodes. The invention further aims at providing a method of manufacturing a charge-coupled device with an efficient, rapid charge transport.
A charge-coupled device of the type mentioned in the opening paragraph is characterized in accordance with the invention in that the average doping concentration of impurity atoms of the second conductivity type in the well is lower at the location of the second electrode than at the location of the first electrode. The invention is based on the recognition that a profiled doping profile in the well also enables the electrical field in the charge-transport channel to be influenced. As will become apparent from the description hereinbelow, a suitable doping profile in the well can be obtained in a simple manner without additional process steps.
A method of the type described in the opening paragraph is characterized in accordance with the invention in that in the doping step of the well, a mask is used which locally masks the surface at the location of the charge-transport channel already present or yet to be formed, as a result of which locally in the well, below the charge-transport channel, a lower doping concentration is obtained than in adjoining parts of the well.
Further embodiments are described in the dependent claims.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.